Fuel cells are envisioned as systems for supplying electricity to mass produced automotive vehicles in the future, and for many other applications. A fuel cell is an electrochemical device that converts chemical energy directly into electrical power. Dihydrogen is used as the fuel of fuel cells. The dihydrogen is oxidized and ionized at an electrode of the cell and dioxygen from the air is reduced at another electrode of the cell. This chemical reaction produces water at the cathode, the oxygen being reduced and reacting with the protons. The great advantage of fuel cells is that they do not emit atmospheric pollutants at the point of generation of electricity.
Proton exchange membrane (PEM) fuel cells have properties, as regards their compactness, that are particularly advantageous. Each cell comprises an electrolytic membrane that only allows protons and not electrons to pass. The membrane comprises an anode on a first face and a cathode on a second face, in order to form a membrane electrode assembly (MEA).
At the anode, the dihydrogen is ionized to produce protons that pass through the membrane. The electrons produced by this reaction migrate toward a flow plate, then flow through an electrical circuit that is external to the cell in order to form an electrical current. At the cathode, oxygen is reduced and reacts with the protons to form water.
The fuel cell may comprise a plurality of flow plates, for example made of metal, stacked one on top of the other. The membrane is placed between two flow plates. The flow plates may comprise channels and orifices for guiding reactants and products to/from the membrane. The plates are also electrically conductive in order to collect the electrons generated at the anode. Gas diffusion layers are interposed between the electrodes and the flow plates and make contact with the flow plates.
The flow plates make contact with very acidic solutions. On the cathode side, the plate is subjected to pressurized air in a highly oxidizing environment. On the anode side, the plate makes contact with hydrogen. Under such conditions, the metal plates are subject to corrosion. Corrosion of a plate leads, on the one hand, to the emission of metal ions, which adversely affect the operation of the electrolytic membrane. On the other hand, corrosion of the plate leads to an insulating oxide layer forming on the metal, thereby increasing its contact resistance with the gas diffusion layer. The electrical resistance between the flow plate and the gas diffusion layer is thus increased. These effects decrease the performance of the fuel cell. The flow plates must therefore have a high electrical conductivity while resisting oxidation and corrosion.
If fuel cells are to be produced industrially, the production cost of the various components will have to decrease. In particular, the cost of flow plates is still far too high for mass market use.
To decrease the cost of flow plates, the latter generally take the form of a bipolar plate comprising two flow plates. One industrially tried-and-tested solution consists in punching two metal sheets made of stainless steel and assembling them back to back by laser welding to form flow plates for adjacent cells. The welds are produced in the bottom of channels, the channel bottoms of two back-to-back metal sheets being placed in contact. In order to decrease production costs, the back-to-back metal sheets have the same geometry.
Document US 2006/046130 describes a fuel cell intended to limit the influence of variations in compressive forces on a stack of cells. The bipolar plates are formed by assembling two metal sheets. Each metal sheet contains a relief in order to form gas flow channels. A multitude of adjacent channels extend in a given direction. Two metal sheets are assembled by placing the bottom of certain channels of these sheets in contact and then producing welds in these bottoms.
In practice, membrane electrode assemblies of known fuel cells have a relatively nonuniform operation. This nonuniformity is due to various effects such as variations in gas moisture content between the inlet and outlet of the membrane electrode assembly. This nonuniformity increases current density locally, promoting localized corrosion of the carbon. Moreover, a higher current density is observed at the welds, the welds substantially increasing conductivity locally. Since the welds of stacked bipolar plates are often superposed, a higher current density is observed through the stack in the area of such a superposition of welds.